1. Field of the Invention
This invention relates to power modulators, and more specifically to power modulators having pulse generating modules utilizing primary and secondary windings.
2. Description of Related Art
a. Modulators, General Description and Definitions of Terminology
A modulator is a device which controls the flow of electrical power. When one turns on an electric lamp and turns it off again, one could be said to be modulating the current that feeds the lamp. In its most common form, a modulator delivers a train of high power electrical pulses to a specialized load like a microwave generator. Most of the world""s high power radar sets use modulators to deliver power pulses to a microwave source, which, in turn, feeds the power, in the form of periodic bursts of microwaves, to an antenna structure. Other possible applications of such power modulator are listed in the text below.
In the decades since World War II, the basic structure of power modulators has not changed significantly. A conventional modulator consists of a power supply, which receives power from an AC power line, steps up the voltage, rectifies the power to produce direct current DC power, and is used to deliver energy to a reservoir, usually formed by an energetic capacitor bank. This is necessary because the input power line cannot deliver the peak power that is required, so the reservoir is used to deliver the peak power in small bites of energy, and is replenished or refilled by the DC power supply at a reasonably constant rate with much lower average power.
Part of the energy in this reservoir is then transferred to a second smaller reservoir, usually a pulse-forming network (PFN). The PFN is a network of capacitors and inductors designed to deliver power to a load in the form of a rectangular (flat-topped) pulse with a fast rise and fall-time as compared to the pulse width or duration.
The pulse-forming network (an artificial transmission line or delay line) is then switched to connect it to the primary side of a pulse transformer, usually but not always a voltage step-up transformer. The PFN voltage before switching is V, and the voltage applied to the pulse transformer primary is V/2 or a bit less. This is one disadvantage of the PFN-technology. The pulse transformer turns ratio (voltage step-up ratio) must be twice as large with a PFN as with the present invention.
The PFN discharges completely in a time T (typically a few to a few tens of microseconds), holding a reasonably constant voltage on the pulse transformer primary and producing a reasonably flat output pulse on the transformer secondary. But if a pulse flatness of 0.1 percent or so is required, then the PFN must have a very large number of inductor-capacitor (LC) sections and it will be difficult to adjust. Also, if any component in the PFN should fail, the PFN will require a new adjustment when the new part is installed, as all the parts values and positions are very critical in a PFN.
Having delivered the pulse, the PFN must be recharged completely to voltage V for the next pulse. To maintain a pulse-to-pulse repeatability of a few tenths of one percent, this large charging voltage xe2x80x9cswingxe2x80x9d must occur with great precision. Also, fully charging and fully discharging all the PFN capacitors for each pulse, several hundred to several thousand times per second, puts a heavy strain on the dielectric material in these capacitors, and this forces the capacitors to be designed with very low stress and hence a very low energy density. This makes the PFN a large structure in comparison to the new invention concept, where the capacitors do not discharge and recharge for each pulse and so can have much higher energy density.
To summarize, the disadvantages of prior art modulators are:
The voltages on the primary side of the pulse transformer are high, typically 10 kV or more.
The PFN must be fully charged to the 10-20 kV range for each pulse, and is fully discharged during the pulse, placing high stress on its capacitors.
The PFN capacitors have low energy density for the above reason, so they are quite large in comparison to the lower-stress capacitors used in the new concept.
If a short circuit occurs at the load (as happens frequently with magnetron tubes), there is no way to interrupt the flow of current, as the high voltage PFN switch (a gas-filled thyratron) cannot be turned off until its current falls to zero.
If a component in the PFN fails, it is necessary to re-tune the PFN for optimal pulse shape after the component is replaced. This is laborious and dangerous work, as it must be done with high voltage applied to the PFN.
If a different pulse width is needed, it is necessary to replace and re-tune the PFN structure.
b. Pulse Transformers
The story of the so-called fractional-turn pulse transformer begins with an invention of Nicholas Christofilos which was assigned to the U.S. Government""s Lawrence Livermore National Laboratory (LLNL) in the early 1960s. At that time, the laboratory was named Lawrence Livermore Laboratory or LLL. This invention disclosed a way to use a large number of toroidal (doughnut-shaped) magnetic cores, each core driven by a high voltage pulse generator at several tens of kilovolts (kV) (using a spark-gap switch and a pulse-forming network or PFN) to generate an accelerating potential of several hundred kV to several megavolts (MV) to accelerate a beam of charged particles. The basic idea of this so-called Linear Magnetic Induction (LMI) Accelerator is shown below in FIGS. 1 and 2.
FIG. 1 illustrates a set of toroidal magnetic cores arranged so their central holes surround a straight line, along which the particle beam is to be accelerated.
FIG. 2 shows the LMI structure with more details added; one high voltage (HV) driver system is shown (each core has one) and the particle beam path is indicated.
The key feature of this type of accelerator is that it has an outer surface which is at ground potential. The voltage which drive the individual cores all appear to add in series down the central axis, but do not appear anywhere else. This means the accelerator does not radiate energy to the outside world and is easy to install in a laboratory as it needs no insulation from its surrounding. An 800 kV LMI accelerator was built at LLL in the 1960s (The ASTRON accelerator), and was used for electron-beam acceleration in fusion experiments. A larger LMI machine (FXR, for flash x-ray) was built at that laboratory in the 1970s, and accelerated an electron beam pulse into an x-ray conversion target. FXR was used for freeze-frame radiography of explosions.
The operating principle of the LMI accelerator can be illustrated with the aid of FIG. 3, which shows a cross-section of the machine in a plane that includes the beam axis.
Some rules of the game are needed to discuss the behavior of the multiple-core structure shown in FIG. 3. First, the right-hand rule is needed. This (arbitrary) rule states that if you grasp a conductor with your right hand, with your thumb pointing in the direction of positive current flow, then your fingers will curl around the conductor in the direction of the magnetic flux lines that encircle the conductor. Applying that rule to FIG. 3, the magnetic flux induced in the toroidal magnetic cores will circulate as shown. A xe2x80x9cdotxe2x80x9d is used to indicate flux vectors pointing toward the reader, and an X is used to represent flux vectors pointing away from the reader.
Applying this rule to the particle beam flowing toward the right along the axis of the structure in FIG. 3, one find that the magnetic flux generated by this beam circulates in the direction opposite to the flux induced by the primary current, which is correct. If we think of this as a transformer, and the beam as a short circuit across the secondary winding, then the current in this secondary will flow in a direction to cancel the flux induced by the primary, causing no net flux to be induced in the magnetic cores and thus presenting a short circuit to the primary power source. No flux change in the cores means no voltage on the primary windings, and this is a short circuit by definition. A beam of positively charged particles (protons) would therefore be accelerated toward the right by the structure, and a beam of negatively charged particles (electrons) would be accelerated toward the left.
One now applies another rule of electronics, namely that the voltage induced in a conductor which surrounds a magnetic flux is equal to the rate of change of that magnetic flux. Consider the path ABCD, which surrounds the flux of all five cores. The voltage induced in a wire that follows this path would equal the rate of change of flux in all of the five cores together. But each core is driven by a primary voltage V, so each core has a rate of change of flux equal to V. Therefore, the voltage induced along the path ABCD would be 5V. The structure acts as a voltage step-up transformer. Another rule is that in a transformer, the ratio of secondary voltage to primary voltage equals the ratio of secondary turns to primary turns, so the LMI accelerator of FIG. 3 has an effective turns ratio of five, yet the path ABCD represents only a single turn. So the primary must be ⅕ of a turn, hence the LMI accelerator can be thought of as a transformer with a fractional-turn primary.
c. Additional Related Art
FIG. 4 is a sketch of the pulse transformer connection that is disclosed in U.S. Pat. No. 5,905,646, Crewson, et.al, May 18, 1999. For simplicity, two pulse generating modules are shown. Each module, as can be seen, drives a single-turn primary (1) that loops around one of the two magnetic cores. Each module contains a capacitor charged to voltage V, and has a catch diode or inverse diode D, to protect the switch from a destructive xe2x80x9cbackspikexe2x80x9d of voltage when the switch turns off.
The above given U.S. Pat. No. 5,905,646 emphasizes the idea that each module drives an independent turn in the primary structure. This is done to assure that all the module switches would conduct the same current. But this restriction opens the modulator up to a potentially destructive fault mode. To understand the destructive fault, assume that the two switches in FIG. 4 do not conduct at precisely the same time. If the upper switch begins conducting a fraction of a microsecond earlier than the lower switch (or vice versa), trouble strikes. If this occurs, the upper magnetic core carries flux in the direction shown (flowing down into the page at the X and up out of the page the dot symbol). This flux induces a current to flow in the secondary and load as shown. No flux is yet present in the lower core, as its module switch is still non-conducting. But the current flowing in the secondary winding will induce a flux in the lower core opposite the direction indicated. This flux will induce a current in the lower module connection as shown, and this current causes the diode D in the lower module to conduct.
Now, when the switch in the lower module does conduct, the applied voltage back-biases the lower diode (which is conducting) and this forces the diode to turn off. Turning off a conducting diode in a few nanoseconds when it is conducting a high current will usually destroy the diode. When the diode is destroyed, it becomes a short-circuit. This short circuit then draws an almost unlimited current through the lower module switch and destroys the switch.
It is an object of the present invention to eliminate the above-mentioned drawback in the prior art modulator as in U.S. Pat. No. 5,905,646. Another object of the present invention is to provide a power modulator with a primary winding connection that eliminates the need to have an equal number of pulse generating modules and primary windings. It is yet another object of the present invention to provide a power modulator that eliminates the earlier mentioned drawbacks in prior art power modulators in that:
the voltages on the primary side of the pulse transformer are low, typically 1 kV or less;
there is no PFN, hence all the disadvantages of the PFN are avoided because the modulator switches are semiconductors like IGBTs or MosFets, which can be turned off with current flowing in them to terminate the pulse;
the energy storage capacitors do not discharge more than a few percent during a pulse, so their energy density can be much higher than for PFN capacitors;
if a short circuit occurs at the load, this can be detected by observing the sudden drop in load voltage, generating a signal that trips a fast comparator which removes the low-voltage gate pulses from the semiconductor switches, terminating the pulse (prior art modulators use overcurrent detectors for this purpose, which are much less fast in operation and allow much higher current to flow before turn-off); and
if a different pulse width is needed, this can be provided by simply changing the timing of the solid-state switch triggers, an operation that occurs at low voltage and can be done from the computer control station, allowing simple electronic adjustment of pulse width.
These advantages lead to the consequent advantages of much smaller size and very much longer service life for the solid-state modulator system as compared to the old-technology PFN/Thyratron.
It is still another object of the present invention to provide a power modulator where different pulse generating modules can be turned on or turned off at different times. To be able to turn on or turn off the pulse generating modules at different times is useful to remove overshooting or ringing at the start of the pulse.
For the sake of clarity in the description, the discussion will be limited to the case of two pulse generating modules. This is by no means a restriction of the invention, which on the contrary works with any number of pulse generating modules.
The addition of two more single turns as shown in FIG. 5 will completely eliminate the earlier mentioned over-voltage failure mode, and will simultaneously eliminate the restriction of having equal numbers of pulse generating modules and primary windings. In prior-art modulators, built under U.S. Pat. No. 5,905,646, one is restricted to having one pulse generating module per primary winding and at least one primary winding per core section. With the present invention, this restriction is removed, and one can use any number of modules. The prior art requires that each core section be driven by the same number of modules. But with the present invention, we can use any number of modules and still provide the same drive signal to each core. This is a strong economic advantage in favor of the present invention.
When the wires (11) in FIG. 5 are added, then whichever module switch is first to conduct will control the circuit until the other switch closes. If the upper switch conducts before the lower one, then the upper module will drive flux in both cores, not just the upper core. This will prevent the lower diode from being drawn into conduction, as this diode will be back-biased. The effect is nearly the same as if all the primaries were connected in parallel, in that the xe2x80x9cearlyxe2x80x9d switch will impose a positive voltage on all the diodes in the xe2x80x9clatexe2x80x9d modules.
One could further simplify things in fact, and connect all the primaries in parallel. This is not obvious, but a look at FIG. 4 will help one see that it works. If all the switches do in fact close at the same time, then there is no voltage between points P and R in the figure. If the points Q and T are connected together, there is no voltage between these points either. If there is no voltage between P and R, then it is possible to connect these points together as well, without causing any additional current to flow, so the circuit will work just the same with all the primaries connected together.
Both of the above connections will in fact serve to equalize the module currents, where the independent connection claimed in U.S. Pat. No. 5,905,646 does not achieve this result. This is because for the first time, it is guaranteed that the load impedance presented to all the pulse generating modules is exactly the same. In the prior art, this is not guaranteed. Of the two primary connections that are given above, the one shown in FIG. 5 is to be preferred over the idea of simply connecting all the primaries in parallel, since with all the primaries in parallel, any diode failure in any module will draw all the current from all the modules into the fault, and this could be quite destructive to the switches. The connection of FIG. 5 eliminates this possibility.